Microfluidic-based systems are becoming widely used in biological and chemical analysis applications. Traditionally, flow cytometry has been used to separate or sort a cell or particle of interest from a heterogeneous population. For example, in conventional flow cytometry, a mixture or cells or particles is hydrodynamically focused using a sheath fluid. The cells or particles, which may be labeled with a fluorescent label or the like, is then interrogated using, for example, a laser or other optical apparatus to identify particular cells or particles of interest within the stream. The cells or particles of interest can then be deflected downstream of the interrogation region into an appropriate collection chamber or the like by using high-voltage electrical plates. For example, the cell or particle contained within the droplet of carrier fluid may be positively or negatively charged which can then be attracted (or repulsed) by the charged electrical plates. This causes movement of the droplets into the proper collection chamber.
More recently, various microfluidic-based sorting schemes have been envisioned to sort cells. For example, Fu et al. discloses a microfabricated fluorescence-activated cell sorter that uses electrokinetic flow to sort bacteria and particles. See Fu et al., A microfabricated fluorescence-activated cell sorter, Nature Biotechnology, 17, 1109-111 (1999). U.S. Pat. No. 6,936,811 discloses a microfluidic sorting device that uses a moving optical gradient to sort particles or cells based on their dielectric properties. Still others have disclosed the use of microfabricated electrodes to separate cells using dielectrophoretic/gravitational field-flow fractionation (DEP/G-FFF). See Yang et al., Cell separation on microfabricated electrodes using dielectrophoretic/gravitational field-flow fractionation, Anal. Chem., 71(5):911-918 (1999). In the DEP/G-FFF method, cells are “levitated” to different heights according to the balance of the DEP and gravitational forces. In still another strategy, cell trapping arrays have been proposed that “trap” cells at dielectrophoretic (DEP) traps. See Heida et al., Dielectrophoretic trapping of dissociated fetal cortical rat neurons, Biomedical Engineering, IEEE Transactions of Biomedical Engineering, Vol. 48, No. 8, August 2001; Taff et al., A Scalable Row/Column-Addressable Dielectrophoretic Cell-Trapping Array, 9th Intl., Conf. on Miniaturized Sys. For Chemistry and Life Sciences, October 2005.
Unfortunately, many of the proposed sorting schemes set forth above have significant limitations. For instance, DEP/G-FFF based devices which rely on the balance between the DEP force and the gravitation force is heavily dependent on the velocity control of the flow since those cells or particles in the middle of the channel are flushed out first because of the parabolic flow profile created within the channel. Also, this method suffers from poor discrimination since the particles/cells located at the sides of the microchannel can be eluted along with the “faster” fraction located within the central region of the channel. In addition, devices using DEP/G-FFF or trapping sort cells or particles temporally (e.g., a time-based approach) making throughput low. Because of this, complicated valves and pumps are needed if this type of separation approach were integrated with other sample preparation steps.
There thus is a need for a device and method that is capable of sorting particles and cells using a spatial approach. Namely, heterogeneous mixtures of cells and/or particles may be automatically directed to downstream channels, branches, or collection chambers without the need for ancillary pumps or valves that control flow patterns. In this regard, the sorting device may be integrated into a microfluidic-based total analysis system that includes other process steps like sample preparation. The device should also permit the sorting of heterogeneous mixtures of cells and/or particles without the need of any fluorescent labels or biomarkers.